Let me be a little more specific...maybe you weren't paying attention in my last posts.
The three biggest mistakes the first time aquariums tend to make are:
1. Over stocking the tank (too many/too large of fish for the size of tank)
2. Over-cleaning. If you thoroughly clean everything, you remove all that good bacteria, your tank will never cycle, and your fish will always be in water that has high levels of ammonia--they can only live in those conditions for so long until they DIE.
3. Over feeding. Only feed enough food for your fish to consume in about 3 minutes time. After three minutes, remove all excess food. That food you leave will rot and cause excess ammonia (TOXIC).
You don't need to clean your plants right now, next week, or realistically even next month. Only clean them when they LOOK really dirty. Then, don't clean them all at once--when you clean them, you are removing that helpful bacteria (which you don't want to do)...So only clean one at a time--maybe one per week. This way, you don't wipe out the WHOLE colony of bacteria but just a little bit.
(Enter cleaning the tank too much!) If you remove all that bacteria, ammonia levels spike and your fish dies. Plain and simple. This is a common mistake for "newbies"
The same is true for the gravel. A simple gravel vacuum (with a siphon) is more than sufficient. Don't remove gravel and rinse it out. The siphon will get out the big chunks of waste, and the decaying food that is missed...the good bacteria should pretty much take care of the rest.
IMO, do nothing more than frequent water changes and occasionally vacuum out the gravel. Don't take anything out of the tank to clean it, don't rinse out the gravel, and don't put in new filter media....Give your tank several weeks for the nitrogen cycle to complete!!!
I CANNOT EMPHASIZE THIS ENOUGH!!!
As for the light...it's fine to turn them off at night. Fish need their rest, too. Remember, goldfish are found in nature. They live in sunlight and moonlight just like everything else...so having the light off at night is natural for them, just as it is for you.
DO NOT add any more fish. You tank is full...in fact, you have a fish that is too big for that tank. That is NOT just my opinion--ask any serious goldfish enthusiast and there will tell you ANY goldfish should have at least 20 gallons of water (for each fish!). Adding more fish will just make your water less inhabitable, cause more stress, and eventually end up stunting and/or killing your fish. NOT GOOD!
An overstocked tank is MUCH harder to take care of, and keep healthy, than a tank that is under stocked or appropriately stocked.
The advice at local pet shops is hit and miss. Again, they're mostly there to make money--sure there are some good, honest, helpful shops out there...but don't expect the best advice from a large chain store. Why not? Because if your fish dies, you come back and buy more fish. They win, you lose. If their advice is not the best, and your fish end up sick or dead...you come back to buy medications or more fish. They win, you lose. They want you to come back and buy more, plain and simple.
If you want good, solid advice...talk to some local enthusiasts that have been successfully been keeping fish for a long period of time, or find a good, honest, successful, mom and pop type fish store..they will tell you the TRUTH about fish care. They have experience the good and the bad, and can really help you AND YOUR PETS out.
I think I've pretty much laid it out on the table. I'm just trying to help. If you choose to disregard my advice and add more fish, or over-clean your tank because you'd rather listen to the lady at the chain store, just trying to make a living, then so be it.
I wish you and your goldy the best, and hope you can both live long, healthy lives.
